Toxoplasma gondii is protozoan parasite of many warm-blooded animals, including humans in whom it can cause severe infection of the central nervous system. Disease is most serious in the developing fetus and in persons who are immunocompromised by HIV infection or other circumstances (transplant recipients, etc.). The extreme environmental stability and oral infectivity of the oocyst form of Toxoplasma have also led to it being designated a significant threat through bioterrorism. Toxoplasma is an obligate intracellular parasite capable of infecting almost any nucleated cell in its already broad host range. It exists in two forms during its asexual growth: the rapidly dividing tachyzoite and the more slowly growing encysted form, the bradyzoite. This application addresses the molecular basis of the interaction between the parasite and the host animal as a whole and between the parasite and the individual host cells in which it resides. The focus of the proposed work is on two subcellular "compartments" within the parasite: its surface, where a family of closely related proteins are found and the rhoptries which are specialized secretory organelles involved in the invasion process and subsequent intracellular life. As regards the surface, the proposal addresses the finding that the plasma membrane is covered by a family of closely related antigens. These are hypothesized to interact with related host cell molecules in the many different animals and/or tissues that the parasite invades. They could also differ in order to manipulate the immune response in such a way that the infection becomes chronic. Experiments are described that use a combination of structural biology and molecular genetics to determine the function of this diverse set of proteins. As regards, the rhoptries, the goals are to determine the identities of their protein constituents and to understand how these proteins contribute to co-opting the host cell for the parasite's own purposes. A combination of proteomics and molecular genetics will be used to dissect this compartment's function. Combined, this effort will substantially enhance our understanding of the host-pathogen interaction for this important pathogen and move us closer to the goal of more effective prevention and treatment strategies.